reames added a comment.
We uncovered another functional issue with this patch, or at least, the
interaction of this patch and other parts of LLVM. In our support for
STATEPOINT, PATCHPOINT, and STACKMAP we use N-byte nop sequences for regions of
code which might be patched out. It's important that these regions are exactly
N bytes as concurrent patching which doesn't replace an integral number of
instructions is ill-defined on X86-64. This patch causes the N-byte nop
sequence to sometimes become (N+M) bytes which breaks the patching. I believe
that the XRAY support may have a similar issue.
More generally, I'm worried about the legality of arbitrarily prefixing
instructions from unknown sources. In the particular example we saw, we had
something along the following:
.Ltmp0:
.p2align 3, 0x90
(16 byte nop sequence)
.Ltmp3:
jmp *%rax
In addition to the patching legality issue above, padding the nop sequence does
something else interesting in this example. It changes the alignment of Ltmp3.
Before, Ltmp3 was always 8 byte aligned, after prefixes are added, it's not.
It's not clear to me exactly what the required semantics here are, but we at
least had been assuming the alignment of Ltmp3 was guaranteed in this case.
(That's actually how we found the patching issue.)
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https://reviews.llvm.org/D70157/new/
https://reviews.llvm.org/D70157
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